Anti-skid control system for automotive brake system with sample control for sampling input timing of sensor signal pulses with required process identification and method for sampling

ABSTRACT

An anti-skid control system, according to the present invention, samples the input timing of the first pulse of each group of a given number of sensor signal pulses, which given number is so adjusted that the difference between the durations of successive pulse groups is great enough to accurately perform the anti-skid control operation based thereupon. Each of the sampling input timing values is provided a label identifying the required processing operations for performing the anti-skid control. The anti-skid control system performs the sampling operation as an interrupt routine intermittently interrupting a main job which derives a control signal for increasing, decreasing or holding constant the fluid pressure applied to the wheel cylinder. The main job contains a plurality of different routines for processing the sampled input data, one of which is to be identified and performed in accordance with the identification in the label. A count of the occurrences of interruption is incremented whenever an interrupt occurs and is decremented upon completion of each cycle of the main job. The sampling routine is performed in response to the first of each group of the given number of sensor signal pulses. When the difference between the durations of successive pulse groups is less than a predetermined value, then the number of sensor signals to be ignored before sampling the input timing of the first pulse of the next group is increased until the difference becomes greater than the predetermined value.

BACKGROUND OF THE INVENTION

The present invention relates generally to an anti-skid control systemfor an automotive hydraulic brake system for controlling brakingpressure in order to hodl wheel-to-ground slip rate to within a givenrange. More specifically, the invention relates to amicroprocessor-based anti-skid brake control system including a samplecontrol routine for sampling the input timing of sensor signal pulsesand a plurality of processing operations which process the input timingdata in respectively different ways.

As is well known, in anti-skid control, the braking force applied towheel cylinders is so adjusted that the peripheral speed of the wheelsduring braking is held to a given ratio, e.g. 80%, of the vehicle speed.Such a practice has been believed to be effective, especially when roadconditions and other factors are taken into consideration. Throughoutthe accompanying disclosure, the ratio of wheel peripheral speed tovehicle speed will be referred to as "slip rate" or "slip ratio".

U.S. Pat. No. 4,267,575, issued on May 12, 1981 to Peter BOUNDS,discloses a system, which serves to provide signals to amicrocomputer-based control system from which instantaneous values ofspeed can be computed, includes a wheel-driven alternator which providesan alternating current output whose frequency varies with wheel speed. Asignal processor converts this signal to a series of sensor pulses whosewidth varies inversely with frequency. A sample pulse supplied by amicroprocessor sets the period or length of time during which the sensorpulses are examined for each speed calculation cycle of themicroprocessor. The sample period pulses are AND-gated with ahigh-frequency clock signal and also with the sensor pulses to provide aseries of marker pulses making the up and down excursions of the sensorpulses. The marker pulses occurring in each sample period are counteddirectly in a first counter, and in addition are supplied to a latchcircuit and from thence to an AND gate which responds to the firstmarker pulse in the sample period to count occurrences of the firstcounter exceeding its capacity. A third counter is also connected toreceive the high-frequency clock pulses and counts only the clock pulsesoccurring after the last marker pulse in the sample period. At the endof the sample period, the counts from all three counters are transferredto the microprocessor which uses this information to compute a value forwheel velocity over the sample period. The system continuously providesthe input counts to enable the microprocessor to calculate wheelvelocity over each sample period.

In addition, U.S. Pat. No. 4,315,213, issued on Feb. 9, 1982 to ManfredWOLFF, discloses a method for obtaining an acceleration or decelerationsignal from a signal proportional to speed and apparatus therefor. Themethod for obtaining an acceleration or deceleration signal from asignal proportional to the speed consists of storing the n most recentlyascertained changes in the speed signal in a memory, and uponascertainment of a new change to be stored in memory, erasing the changewhich has been stored the longest, and forming a deceleration oracceleration signal by addition of the stored n changes periodically atintervals of dT. In this method, the occurrence of deceleration oracceleration exceeding the threshold is recognized quickly.

In another approach, U.S. Pat. No. 4,384,330 to Toshiro MATSUDA, issuedon May 17, 1983 discloses a brake control system for controllingapplication and release of brake pressure in order to prevent thevehicle from brake pressure in order to prevent the vehicle fromskidding. The system includes a sensing circuit for determining wheelrotation speed, a deceleration detecting circuit for determining thedeceleration rate of the wheel and generating a signal when thedetermined deceleration rate becomes equal to or greater than apredetermined value, a target wheel speed circuit for determining atarget wheel spaced based on the wheel rotation speed and operative inresponse to detection of a peak in the coefficient of friction betweenthe vehicle wheel and the road surface, and a control circuit forcontrolling application and release of brake fluid pressure to wheelcylinders for controlling the wheel deceleration rate. The wheelrotation speed sensing circuit detects the angular velocity of the wheelto produce alternating current sensor signal having a frequencycorresponding to the wheel rotation speed. The wheel rotation speedsensor signal value is differentiated to derive the deceleration rate.

Another approach for deriving acceleration has been disclosed in U.S.Pat. No. 3,943,345 issued on Mar. 9, 1976 to Noriyoshi ANDO et al. Thesystem disclosed includes a first counter for counting the number ofpulse signals corresponding to the rotational speed of a rotating body,a second counter for counting the number of pulses after the firstcounter stops counting, and a control circuit for generating an outputsignal corresponding to the difference between the counts of the firstand second counters.

In the present invention, another approach has been taken to derive thewheel rotation speed which will be hereafter referred to as "wheelspeed" based on input time data representative of the times at whichwheel speed sensor signal pulses are produced. For instance, by latchinga timer signal value in response to the leading edge of each sensorsignal pulse, the intervals between occurrences of the sensor signalpulses can be measured. The intervals between occurrences of the sensorsignal pulses are inversely proportional to the rotation speed of thewheel. Therefore, wheel speed can be derived by finding the reciprocalof the measured intervals. In addition, wheel acceleration anddeceleration can be obtained by comparing successive intervals anddividing the obtained difference between intervals of the period of timeover which the sensor signals were sampled.

To perform this procedure, it is essential to record the input timing inresponse to every sensor signal pulse. A difficulty is encountered dueto significant variations in the sensor signal intervals according tosignificant variations in the vehicle speed. In recent years, modernvehicles can be driven at speeds in the range of about 0 km to 300 km.Sensor signal intervals vary in accordance with this wide speed range.In particular, when the vehicle is moving at a relatively high speed,the input intervals of the sensor signal pulses may be too short for theanti-skid control system to resolve. As accurate sampling of inputtiming is essential for the proposed approach, errors in the recordedinput time data will cause errors or malfunction of the anti-skid brakecontrol system. One possible source of error in sampling the inputtiming is that of accidentally missing one or more sensor signal pulses.Such errors are particularly likely to occur when the vehicle and wheelspeeds are relatively high and therefore the intervals between adjacentsensor signal pulses are quite short.

U.S. Pat. No. 4,408,290, issued on Oct. 4, 1983 to the common inventorof this invention is intended to perform the foregoing input time datasampling for use in calculation of acceleration and deceleration. In thedisclosure of the applicant's prior invention, an acceleration sensoracts on the variable-frequency pulses of a speed sensor signal torecognize any variation of the pulse period thereof and to produce anoutput indicative of the magnitude of the detected variation to within afixed degree of accuracy. The durations of groups of pulses are held towithin a fixed range by adjusting the number of pulses in each group.The duration of groups of pulses are measured with reference to afixed-frequency clock pulse signal and the measurement periods ofsuccessive groups of equal numbers pulses are compared. If thedifference between pulse group periods is zero or less than apredetermined value, the number of pulses in each group is increased inorder to increase the total number of clock pulses during themeasurement interval. The number of pulses per group is increased untilthe difference between measured periods exceeds the predetermined valueor until the number of pulses per group reaches a predetermined maximum.Acceleration data calculation and memory control procedure are designedto take into account the variation of the number of pulse per group.

The applicant's prior invention is effective for expanding intervals forsampling the input time data of the sensor pulse signals and forenabling the anti-skid control system to resolve variations in the wheelspeeds.

As will be appreciated, in order to derive a control signal forcontrolling braking pressure to be applied to the wheel cylinder, aplurality of arithmetic operations are required, such as deriving theinterval between adjacent sensor signal pulses, deriving differencebetween successive pulse-to-pulse deriving wheel speed, deriving wheelacceleration and deceleration, etc. Depending on the input timing dataof the sensor signal pulses, the processes which must be carried out aredifferent. Therefore, some of the process steps for deriving the controlsignal can be skipped by identifying the required processes relative toeach of the input timing data.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide ananti-skid brake control system which can reduce the number of processsteps to be performed in response to each sensor signal pulse and thusto reduce the time required to derive the control signal.

Another and more specific object of the present invention is to providean anti-skid control system in which each sampled input timing valueitself identifies the required processes, so that unnecessary steps canbe skipped.

In order to accomplish the above-mentioned and other objects, ananti-skid control system, according to the present invention, samplesthe input timing of the first pulse of each group of a given number ofsensor signal pulses, which given number is so adjusted that thedifference between the durations of successive pulse groups is greatenough to accurately perform the anti-skid control operation basedthereupon. Each of the sampling input timing values is provided a labelidentifying the required processing operations for perfoming theanti-skid control. The anti-skid control system performs the samplingoperation as an interrupt routine intermittently interrupting a main jobwhich derives a control signal for increasing, decreasing or holdingconstant the fluid pressure applied to the wheel cylinder. The main jobcontains a plurality of different routines for processing the sampledinput data, one of which is to be identified and performed in accordancewith the identification in the label. A count of the occurrences ofinterruption is incremented whenever an interrupt occurs and isdecremented upon completion of each cycle of the main job. The samplingroutine is performed in response to the first of each group of the givennumber of sensor signal pulses. When the difference between thedurations of successive pulse groups is less than a predetermined value,then the number of sensor signals to be ignored before sampling theinput timing of the first pulse of the next group is increased until thedifference becomes greater than the predetermined value.

According to one aspect of the present invention, an anti-skid brakecontrol system for an automotive brake system comprises a hydraulicbrake circuit including a wheel cylinder for applying braking force to avehicle wheel, a pressure control valve disposed within the hydrauliccircuit and operative to increase fluid pressure in the wheel cylinderin its first position, to decrease the fluid pressure in the wheelcylinder in its second position and to hold the fluid pressure in thewheel cylinder constant in its third position, a wheel speed sensormeans for detecting the rotational speed of the vehicle wheel andproducing sensor signal pulses separated by intervals representative ofthe detected wheel speed, a timer means for producing a timer signalhaving a value indicative of elapsed time, a first means, responsive tothe sensor signal, for sampling the timer signal value to gain a measureof the sensor signal pulse input timing and for storing the sampledtimer signal values, a second means for reading out the stored timersignal values and processing the read-out timer signal values to producea control signal which actuates the pressure control valve to one of thefirst, second and third positions so as to adjust the wheel rotationalspeed toward an optimal relationship with vehicle speed, the secondmeans performing at least one of a plurality of different operations onthe sampled timer signal values, a third means, associated with thefirst means, for providing identification for each sampled timer signalvalue, the identification identifying which of the plurality ofdifferent operations is to be performed on the corresponding sampledtimer signal value, a fourth means, associated with the first, secondand third means and responsive to the sensor signal pulses to interruptoperation of the second means and activate the first and third means forstoring the sampled timer signal value with the identification, and afifth means for controlling the second means to perform the operationidentified by the identification stored with the currently sampled timersignal value.

According to another aspect of the invention, an anti-skid brake controlsystem for an automotive vehicle comprises a hydraulic brake circuitincluding wheel cylinders respectively corresponding to respectivevehicle wheel for applying braking force to the corresponding wheelcylinder, a pressure control valve associated with each of the wheelcylinder for controlling fluid pressure to be applied to thecorresponding wheel cyliner, the pressure control valve being operativeto increase the fluid pressure in the wheel cylinder at its firstposition, to decrease the fluid pressure in the wheel cylinder at itssecond position and to maintain the fluid pressure in the wheel cylinderat a constant value at its third position, a wheel speed sensor meansfor detecting rotational speed of vehicle wheel to sequentially outputpulse form sensor signals having pulse intervals respectivelycorresponding to instantaneous wheel rotation speed, a timer means forsequentially outputting a timer signal having a value indicative ofmeasured period of time, a controller operative in a sampling operationfor receiving the sensor signal pulses and the timer signal for samplinginput time data at every given number of sensor signals input and in anarithmetic operation for detecting instantaneous slip rate at thecorresponding wheel and acceleration and deceleration of thecorresponding wheel based on the sampled input time data for controllingvalve position of the pressure control valve at a predetermined patternfor optimizing braking characteristics, which arithmetic operationcontaining a plurality of mutually different processes, the controllerinterrupting the arithmetic operation in response to the sensor signalto be sampled to perform sampling operation for sampling the input timedata of the corresponding sensor signal when the difference exceeds thepredetermined value, and the controller providing a label to each inputtime data, which label identifies one of the process in the arithmeticoperation to be performed with respect to the corresponding input timedata.

According to a further apect of the invention, a method for anti-skidcontrolling an automotive hydraulic brake system including a wheelcylinder and a pressure control valve, fluid pressure in the wheelcylinder being increased as the pressure control valve being placed atits first position being decreased as the pressure control valve beingplaced at its second position and being maintained at substantiallyconstant value at a third position of the pressure control valve, themethod comprising the steps of detecting wheel rotation speed tosequentially produce pulse form sensor signals having intervals betweenmutually adjacent sensor signals representative of the detected wheelspeed, sequentially producing a timer signal having a value indicativeof measured period of time, sampling the timer signal value in responseto every given number of the sensor signals to store the timer signalvalue as input time data of the corresponding sensor signal, and givingsequential numbes for respective sampled input data, adjusting the givennumber to determine a sample mode so as to a difference of mutuallyadjacent signal-to-signal intervals is greater than a predeterminedvalue, performing arithmetic operation for processing the input timedata of the sensor signals for detecting slip data and wheelacceleration and deceleration for controlling the pressure control valvepositions according to a predetermined pattern for optimizing brakingcharacteristics at the vehicle wheel, the arithmetic operationcontaining a plurality of different processings to be identified andperformed in accordance with the sequential number given to the inputtime data and interrupting the arithmetic operation in response to theevery given number of sensor signals for performing operation forsampling the timer signal value and storing the sampled timer signalvalue as the input time data relative to the input sensor signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of thepreferred embodiments of the present invention, which, however, shouldnot be taken to limit the invention to the specific embodiments, but arefor explanation and understanding only.

FIG. 1 is a schematic block diagram of the general design of thepreferred embodiment of an anti-skid brake control system according tothe present invention;

FIG. 2 is a perspective illustration of the hydraulic circuits of theanti-skid brake system according to the present invention;

FIG. 3 is a circuit diagram of the hydraulic circuits performing theanti-skid control according to the present invention;

FIG. 4 is an illustration of the operation of an electromagnetic flowcontrol valve employed in the hydraulic circuit, which valve has beenshown in an application mode for increasing the fluid pressure in awheel cylinder;

FIG. 5 is a view similar to FIG. 4 but of the valve in a hold mode inwhich the fluid pressure in the wheel cylinder is held at asubstantially constant value;

FIG. 6 is a view similar to FIG. 4 but of the valve in a release mode inwhich the fluid pressure in the wheel cylinder is reduced;

FIG. 7 is a perspective view of a wheel speed sensor adapted to detectthe speed of a front wheel;

FIG. 8 is a side elevation of a wheel speed sensor adapted to detect thespeed of a rear wheel;

FIG. 9 is an explanatory illustration of the wheel speed sensors ofFIGS. 7 and 8;

FIG. 10 shows the waveform of an alternating current sensor signalproduced by the wheel speed sensor;

FIG. 11 is a timing chart for the anti-skid control system;

FIG. 12 is a block diagram of the preferred embodiment of a controllerunit in the anti-skid brake control system according to the presentinvention;

FIG. 13 is a flowchart of a main program of a microcomputer constitutingthe controller unit of FIG. 12;

FIG. 14 is a flowchart of an interrupt program executed by thecontroller unit;

FIG. 15 is a flowchart of a main routine in the main program of FIG. 13;

FIG. 16 is an explanatory diagram of the input timing sampling modes andvariations thereof;

FIG. 17 is another explanatory diagram of the sampling timing of thesensor pulse input timing;

FIG. 18 is a diagram of the period of time during which sensor pulsesare sampled in accordance with the present invention, which period oftime is compared with that used in the typical prior art;

FIG. 19 is a flowchart of a sample control program executed by thecontroller unit;

FIG. 20 is a flowchart of a timer overflow program executed periodicallyas an interrupt program of the main program;

FIG. 21 is a graph of the variation of a counter value of a clockcounter in the preferred embodiment of controller unit;

FIG. 22 is a timing chart of the timer overflow which is shown inrelation to the value of the timer overflow interrupt flag;

FIG. 23 is a flowchart of an output calculation program for deriving EVand V signals for controlling operation mode of the electromagneticvalve according to the valve conditions of FIGS. 4, 5 and 6;

FIGS. 24 and 25 are diagrams of execution timing of the outputcalculation program in relation to the main program;

FIG. 26 is a table determining the operation mode of the actuator 16,which table is accessed in terms of the wheel acceleration anddeceleration and the slip rate;

FIG. 27 is a flowchart of the main routine similar to FIG. 15, butmodified according to a proposed modification;

FIG. 28 shows the timing of calculations of the wheel speed and thewheel acceleration and deceleration carried out by the modified mainroutine of FIG. 27;

FIG. 29 is a timing chart of calculation similar to FIG. 28 but for aalso modified form;

FIG. 30 shows the variation of wheel speed and wheel acceleration anddeceleration as anti-skid brake control is performed according to thepresent invention; and

FIG. 31 is a block diagram of another embodiment of the controller unitin the preferred embodiment of the anti-skid brake control systemaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This application is one of eighteen mutually related co-pending patentapplications in the United States, filed on the same day. All of theeighteen applications have been filed by the common applicant to thisapplication and commonly assigned to the assignee of this application.The other seventeen applications are identified below:

    __________________________________________________________________________    Basic Japanese                                                                Patent Appln No.                                                              Serial No.     Title of the Invention                                         __________________________________________________________________________    Showa 58-70891 AN AUTOMOTIVE ANTI-SKID BRAKE                                  601,326, filed April 17, 1984                                                                CONTROL SYSTEM WITH SAMPLING INPUT                                            TIME DATA OF WHEEL SPEED SENSOR                                               SIGNALS                                                        Showa 58-70892 METHOD AND SYSTEM FOR SAMPLING INPUT                           601,375, filed April 17, 1984                                                                TIME DATA FOR WHEEL SPEED SENSOR IN                                           AN AUTOMOTIVE ANTI-SKID BRAKE                                                 CONTROL SYSTEM                                                 Showa 58-70893 AUTOMOTIVE ANTI-SKID CONTROL SYSTEM                            601,325, filed April 17, 1984                                                                WITH CONTROL OF SAMPLING OF INPUT                                             TIME DATA OF WHEEL SPEED SENSOR                                               SIGNALS AND METHOD THEREFOR                                    Showa 58-70895 ANTI-SKID BRAKE CONTROL SYSTEM                                 601,294, filed April 17, 1984                                                                INCLUDING A PROCEDURE OF SAMPLING OF                                          INPUT TIME DATA OF WHEEL SPEED                                                SENSOR SIGNALS AND METHOD THEREFOR                             Showa 58-70896 ANTI-SKID BRAKE CONTROL SYSTEM                                 601,344, filed April 17, 1984                                                                INCLUDING WHEEL DECELERATION CALCU-                                           LATION WITH SHORTER LAB-TIME AND                                              METHOD FOR PERFORMING CALCULATION                              Showa 58-70897 ANTI-SKID BRAKE CONTROL SYSTEM WITH                            601,388, filed April 17, 1984                                                                SAMPLE CONTROL OF SENSOR SIGNAL                                               INPUT TIME DATA, AND METHOD THEREFOR                           Showa 58-70898 ANTI-SKID BRAKE CONTROL SYSTEM WITH                            601,337, filed April 17, 1984                                                                CONTROL OF SAMPLING TIMING OF INPUT                                           TIMING VALUES OF WHEEL SPEED SENSOR                                           SIGNAL PULSES                                                  Showa 58-70899 ANTI-SKID BRAKE CONTROL SYSTEM FOR                             601,330, filed April 17, 1984                                                                AUTOMOTIVE VEHCLE                                              Showa 58-70900 ANTI-SKID BRAKE CONTROL SYSTEM WITH                            601,364, filed April 17, 1984                                                                REDUCED DURATION OF WHEEL ACCELER-                                            ATION AND DECELERATION CALCULATION                             Showa 58-84088 ANTI-SKID BRAKE CONTROL SYSTEM WITH                            601,363, filed April 17, 1984                                                                OPERATIONAL MODE CONTROL AND METHOD                                           THEREFOR                                                       Showa 58-84087 ANTI-SKID BRAKE CONTROL SYSTEM WITH                            & 58-84091     OPERATION CONTROL FOR A PRESSURE                               601,329, filed April 17, 1984                                                                REDUCTION FLUID PUMP IN HYDRAULIC                                             BRAKE CIRCUIT                                                  Showa 58-84082 METHOD AND SYSTEM FOR DERIVING WHEEL                           601,318, filed April 17,1984                                                                 ROTATION SPEED DATA FOR AUTOMOTIVE                                            ANTI-SKID CONTROL                                              Showa 58-84085 METHOD AND SYSTEM FOR DERIVING WHEEL                           601,345, filed April 17, 1984                                                                ACCELERATION AND DECELERATION IN                                              AUTOMOTIVE ANTI-SKID BRAKE CONTROL                                            SYSTEM                                                         Showa 58-84092 ANTI-SKID BRAKE CONTROL SYSTEM AND                             601,293, filed April 17, 1984                                                                METHOD FEATURING VEHICLE BATTERY                                              PROTECTION                                                     Showa 58-84081 METHOD AND SYSTEM FOR DERIVING WHEEL                           601,327, filed April 17, 1984                                                                ROTATION SPEED DATA FOR AUTOMOTIVE                                            ANTI-SKID CONTROL                                              Showa 58-84090 ANTI-SKID BRAKE CONTROL SYSTEM                                 601,258, filed April 17, 1984,                                                               INCLUDING FLUID PUMP AND DRIVE                                 now Patent No. 4,569,560                                                                     CIRCUIT THEREFOR                                               issued February 11, 1986                                                      Showa 58-102919                                                                              ANTI-SKID BRAKE CONTROL SYSTEM WITH                            & 58-109308    A PLURALITY OF INDEPENDENTLY                                   601,295, filed April 17, 1984                                                                OPERATIVE DIGITAL CONTROLLERS                                  __________________________________________________________________________

Disclosures of other seventeen applications as identified above arehereby incorporated by reference for the sake of disclosure.

Referring now to the drawings, particularly to FIG. 1, the preferredembodiment of an anti-skid control system according to the presentinvention includes a control module 200 including a front-leftcontroller unit (FL) 202, a front-right controller unit (FR) 204 and arear controller unit (R) 206. The controller unit 202 comprises amicroprocessor and is adapted to control brake pressure applied to afront left wheel cylinder 30a of a front left hydraulic brake system 302of an automotive hydraulic brake system 300. Similarly, the controllerunit 204 is adapted to control brake pressure applied to the wheelcylinder 34a of a front right wheel (not shown) in the front righthydraulic brake system 304 and the controller unit 206 is adapted tocontrol brake pressure applied to the rear wheel cylinders 38a of thehydraulic rear brake system 306. Respective brake systems 302, 304 and306 have electromagnetically operated actuators 16, 18 and 20, each ofwhich controls the pressure of working fluid in the corresponding wheelcylinders. By means of the controlled pressure, the wheel cylinders 30a,34a and 38a apply braking force to brake disc rotors 28, 32 and 36mounted on the corresponding wheel axles for rotation with thecorresponding vehicle wheels via brake shoe assemblies 30, 34 and 38.

Though the disclosed brake system comprises disc brakes, the anti-skidcontrol system according to the present invention can also be applied todrum-type brake systems.

The controller units 202, 204 and 206 are respectively associated withactuator drive circuits 214, 216 and 218 to control operations ofcorresponding actuators 16, 18 and 20. In addition, each of thecontroller units 202, 204 and 206 is connected to a corresponding wheelspeed sensor 10, 12 and 14 via shaping circuits 208, 210 and 212incorporated in the controller 200. Each of the wheel speed sensors 10,12 and 14 is adapted to produce an alternating-current sensor signalhaving a frequency related to or proportional to the rotation speed ofthe corresponding vehicle wheel. Each of the A-C sensor signals isconverted by the corresponding shaping circuit 208, 210 and 212 into arectangular pulse signal which will be hereafter referred to as "sensorpulse signal". As can be appreciated, since the frequency of the A-Csensor signals is proportional to the wheel speed, the frequency of thesensor pulse signal should correspond to the wheel rotation speed andthe pulse intervals thereof will be inversely proportional to the wheelrotation speed.

The controller units 202, 204 and 206 operate independently andcontinuously process the sensor pulse signal to derive control signalsfor controlling the fluid pressure in each of the wheel cylinders 30a,34a and 38a in such a way that the slip rate R at each of the vehiclewheels is optimized to shorten the distance required to stop thevehicle, which distance will be hereafter referred to as "brakingdistance".

In general, each controller unit 202, 204 and 206 monitors receipt ofthe corresponding sensor pulses so that it can derive the pulse intervalbetween the times of receipt of successive sensor pulses. Based on thederived pulse interval, the controller units 202, 204 and 206 calculateinstantaneous wheel speed V_(w) and instantaneous wheel acceleration ordeceleration a_(w). From these measured and derived values, a targetwheel speed V_(i) is derived, which is an assumed value derived from thewheel speed at which a slip is assumed to be zero or approximately zero.The target wheel speed V_(i) varies at a constant decelerating ratederived from variation of the wheel speed. The target wheel speed thuscorresponds to a vehicle speed which itself is based on variations ofthe wheel speed. Based on the difference between the instantaneous wheelspeed V_(w) and the target wheel speed V_(i), a slip rate R is derived.The controller units 202, 204 and 206 determine the appropriateoperational mode for increasing, decreasing or holding the hydraulicbrake pressure applied to the wheel cylinders 30a, 34a and 38a. Thecontrol mode in which the brake pressure is increased will be hereafterreferred to as "application mode". The control mode in which the brakepressure is decreased will be hereafter referred to as "release mode".The mode in which the brake pressure is held essentially constant willbe hereafter referred to as "hold mode". The anti-skid control operationconsists of a loop of the application mode, hold mode, release mode andhold mode. This loop is repeated throughout the anti-skid brake controloperation cyclically. One cycle of the loop of the control variationwill be hereafter referred to as "skid cycle".

FIG. 2 shows portions of the hydraulic brake system of an automotivevehicle to which the preferred embodiment of the anti-skid controlsystem is applied. The wheel speed sensors 10 and 12 are respectivelyprovided adjacent the brake disc rotor 28 and 32 for rotation therewithso as to produce sensor signals having frequencies proportional to thewheel rotation speed and variable in accordance with variation of thewheel speed. On the other hand, the wheel speed sensor 14 is providedadjacent a propeller shaft near the differential gear box or drivepinion shaft 116 for rotation therewith. (See FIG. 8) Since the rotationspeeds of the left and right rear wheels are free to vary independentlydepending upon driving conditions due to the effect of the differentialgear box 40, the rear wheel speed detected by the rear wheel speedsensor 14 is the average of the speeds of the left and right wheels.Throughout the specification, "rear wheel speed" will mean the averagerotation speed of the left and right rear wheels.

As shown in FIG. 2, the actuator unit 300 is connected to a master wheelcylinder 24 via primary and secondary outlet ports 41 and 43 thereof andvia pressure lines 44 and 42. The master wheel cylinder 24 is, in turn,associated with a brake pedal 22 via a power booster 26 which is adaptedto boost the braking force applied to the brake pedal 22 before applyingsame to the master cylinder. The actuator unit 300 is also connected towheel cylinders 30a, 34a and 38a via brake pressure lines 46, 48 and 50.

The circuit lay-out of the hydraulic brake system circuit will bedescribed in detail below with reference to FIG. 3 which is only anexample of the hydraulic brake system to which the preferred embodimentof the anti-skid control system according to the present invention canbe applied, and so it should be appreciated that it is not intended tolimit the hydraulic system to the embodiment shown. In FIG. 3, thesecondary outlet port 43 is connected to the inlet ports 16b and 18b ofelectromagnetic flow control valves 16a and 18a, the respective outletports 16c and 18c of which are connected to corresponding left and rightwheel cylinders 30a and 34a, via the secondary pressure lines 46 and 48.The primary outlet port 41 is connected to the inlet port 20b of theelectromagnetic valve 20a, the outlet port 20c of which is connected tothe rear wheel cylinders 38a, via a primary pressure line 50. Theelectromagnetic valves 16a, 18a and 20a also have drain ports 16d, 18dand 20d. The drain ports 16d and 18d are connected to the inlet port 72aof a fluid pump 90 via drain passages 80, 82 and 78. The fluid pump 90is associated with an electric motor 88 to be driven by the latter whichis, in turn, connected to a motor relay 92, the duty cycle of which iscontrolled by means of a control signal from the control module 200.While the motor relay 92 is energized to be turned ON, the motor 88 isin operation to drive the fluid pump 90. The drain port 20d of theelectromagnetic flow control valve 20a is connected to the inlet port58a of the fluid pump 90 via a drain passage 64.

The outlet ports 72b and 58b are respectively connected to the pressurelines 42 and 44 via a return passages 72c and 58c. The outlet ports 16c,18c and 20c of respective electromagnetic flow control valves 16a, 18aand 20a are connected to corresponding wheel cylinders 30a, 34a and 38avia braking lines 46, 48 and 50. Bypass passages 96 and 98 are providedto connect the braking pressure lines 46 and 48, and 50 respectively tothe pressure lines 42 and 44, bypassing the electromagnetic flow controlvalves.

Pump pressure check valves 52 and 66 are installed in the pressure lines42 and 44. Each of the pump pressure check valves 66 and 52 is adaptedto prevent the working fluid pressurized by the fluid pump 90 fromtransmitting pressure surges to the master cylinder 24. Since the fluidpump 90 is designed for quick release of the braking pressure in thebraking pressure lines 46, 48 and 50 and thus releasing the wheelcylinders 30a, 34a and 38a from the braking pressure, it is driven uponrelease of the brake pedal. This would result in pressure surges in theworking fluid from the fluid pump 90 to the master cylinder 24 if thepump pressure check valves 66 and 52 were not provided. The pumppressure check valves 66 and 52 serve as one-way check valves allowingfluid flow from the master cylinder 24 to the inlet ports 16b, 18b and20b of the electromagnetic valves 16a, 18a and 20a. Pressureaccumulators 70 and 56 are installed in the pressure lines 42 and 44,which pressure accumulators serve to accumulate fluid pressure generatedat the outlet ports 72b and 58b of the fluid pump 90 while the inletports 16b, 18b and 20b are closed. Toward this end, the pressureaccumulators 70 and 56 are connected to the outlet ports 72b and 58b ofthe fluid pump 90 via the return passages 72c and 58c. Outlet valves 68and 54 are one-way check valves allowing one-way fluid communicationfrom the fluid pump to the pressure accumulators. These outlet valves 68and 54 are effective for preventing the pressure accumulated in thepressure accumulators 70 and 56 from surging to the fluid pump when thepump is deactivated. In addition, the outlet valves 68 and 54 are alsoeffective to prevent the pressurized fluid flowing through the pressurelines 42 and 44 from flowing into the fluid pump 90 through the returnpassages 72c and 58c.

Inlet check valves 74 and 60 are inserted in the drain passages 78 and64 for preventing surge flow of the pressurized fluid in the fluid pump90 to the electromagnetic flow control valves 16a, 18a and 20a after thebraking pressure in the wheel cylinders is released. The fluid flowingthrough the drain passages 78 and 64 is temporarily retained in fluidreservoirs 76 and 62 connected to the former.

Bypass check valves 85, 86 and 84 are inserted in the bypass passages 98and 96 for preventing the fluid in the pressure lines 42 and 44 fromflowing to the braking pressure lines 46, 48 and 50 without firstpassing through the electromagnetic flow control valves 16a, 18a and20a. On the other hand, the bypass check valves 85, 86 and 84 areadapted to permit fluid flow from the braking pressure line 46, 48 and50 to the pressure lines 42 and 44 when the master cylinder 24 isreleased and thus the line pressure in the pressure lines 42 and 44becomes lower than the pressure in the braking pressure lines 46, 48 and50.

The electromagnetic flow control valves 16a, 18a and 20a arerespectively associated with the actuators 16, 18 and 20 to becontrolled by means of the control signals from the control module 200.The actuators 16, 18 and 20 are all connected to the control module 200via an actuator relay 94, which thus controls the energization anddeenergization of them all. Operation of the electromagnetic valve 16ain cooperation with the actuator 16 will be illustrated with referenceto FIGS. 4, 5 and 6, in particular illustrating the application mode,hold mode and release mode, respectively.

It should be appreciated that the operation of the electromagneticvalves 18a and 20a are substantially the same as that of the valve 16a.Therefore, disclosure of the valves operations of the electromagneticvalves 18a and 20a is omitted in order to avoid unnecessary repetitionand for simplification of the disclosure.

APPLICATION MODE

In this position, the actuator 16 remains deenergized. An anchor of theelectromagnetic valve 16a thus remains in its initial position allowingfluid flow between the inlet port 16b and the outlet port 16c so thatthe pressurized fluid supplied from the master cylinder 24 via thepressure line 42 may flow to the left front wheel cylinder 30a via thebraking pressure line 46. In this valve position, the drain port 16d isclosed to block fluid flow from the pressure line 42 to the drainpassage 78. As a result, the lin pressure in the braking pressure line46 is increased in proportion to the magnitude of depression of thebrake pedal 22 and thereby the fluid pressure in the left front wheelcylinder 30a is increased correspondingly.

In this case, when the braking force applied to the brake pedal isreleased, the line pressure in the pressure line 42 drops due to returnof the master cylinder 24 to its initial position. As a result, the linepressure in the braking pressure line 46 becomes higher than that in thepressure line 42 and so opens the bypass valve 85 to permit fluid flowthrough the bypass passage 98 to return the working fluid to the fluidreservoir 24a of the master cylinder 24.

In the preferring construction, the pump pressure check valve 66,normally serving as a one-way check valve for preventing fluid flow fromthe electromagnetic valve 16a to the master cylinder 24, becomeswide-open in response to drop of the line pressure in the pressure linebelow a given pressure. This allows the fluid in the braking pressureline 46 to flow backwards through the electromagnetic valve 16a and thepump pressure check valve 66 to the master cylinder 24 via the pressureline 42. This function of the pump pressure check valve 66 facilitatesfull release of the braking pressure in the wheel cylinder 30a.

For instance, the bypass valve 85 is rated at a given set pressure, e.g.2 kg/cm² and closes when the pressure difference between the pressureline 42 and the braking pressure line 46 drops below the set pressure.As a result, fluid pressure approximating the bypass valve set pressuretends to remain in the braking pressure line 46, preventing the wheelcylinder 30a from returning to the fully released position. In order toavoid this, in the shown embodiment, the one-way check valve function ofthe pump pressure check valve 66 is disabled when the line pressure inthe pressure line 42 drops below a predetermined pressure, e.g. 10kg/cm². When the line pressure in the pressure line 42 drops below thepredetermined pressure, a bias force normally applied to the pumppressure check valve 66 is released, freeing the valve to allow fluidflow from the braking pressure line 46 to the master cylinder 24 via thepressure line 42.

HOLD MODE

In this control mode, a limited first value, e.g. 2 A of electriccurrent serving as the control signal is applied to the actuator 16 toposition the anchor closer to the actuator 16 than in the previous case.As a result, the inlet port 16b and the drain port 16d are closed toblock fluid communication between the pressure line 42 and the brakingpressure line 46 and between the braking pressure line and the drainpassage 78. Therefore, the fluid pressure in the braking pressure line46 is held at the level extant at the moment the actuator is operated bythe control signal.

In this case, the fluid pressure applied through the master cylinderflows through the pressure check valve 66 to the pressure accumulator70.

RELEASE MODE

In this control mode, a maximum value, e.g. 5A of electric currentserving as the control signal is applied to the actuator 16 to shift theanchor all the way toward the actuator 16. As a result, the drain port16d is opened to establish fluid communication between the drain port16d and the outlet port 16c. At this time, the fluid pump 90 serves tofacilitate fluid flow from the braking pressure line 46 to the drainpassage 78. The fluid flowing through the drain passage is partlyaccumulated in the fluid reservoir 76 and the remainder flows to thepressure accumulator 70 via the check valves 60 and 54 and the fluidpump 90.

It will be appreciated that, even in this release mode, the fluidpressure in the pressure line 42 remains at a level higher or equal tothat in the braking pressure line 46, so that fluid flow from thebraking pressure line 46 to the pressure line 42 via the bypass passage98 and via the bypass check valve 85 will never occur.

In order to resume the braking pressure in the wheel cylinder (FL) 30aafter once the braking pressure is reduced by positioning theelectromagnetic valve 16a in the release position, the actuator 16 isagain deenergized. The electromagnetic valve 16a is thus returns to itsinitial position to allow the fluid flow between the inlet port 16b andthe outlet port 16c so that the pressurized fluid may flow to the leftfront wheel cylinder 30a via the braking pressure line 46. As set forththe drain port 16d is closed to block fluid flow from the pressure line42 to the drain passage 78.

As a result, the pressure accumulator 70 is connected to the left frontwheel cylinder 30a via the electromagnetic valve 16a and the brakingpressure line 46. The pressurized fluid in the pressure accumulator 70is thus supplied to the wheel cylinder 30a so as to resume the fluidpressure in the wheel cylinder 30a.

At this time, as the pressure accumulator 70 is connected to the fluidreservoir 76 via the check valves 60 and 54 which allow fluid flow fromthe fluid reservoir to the pressure accumulator, the extra amount ofpressurized fluid may be supplied from the fluid reservoir.

The design of the wheel speed sensors 10, 12 and 14 employed in thepreferred embodiment of the anti-skid control system will be describedin detail with reference to FIGS. 7 to 9.

FIG. 7 shows the structure of the wheel speed sensor 10 for detectingthe rate of rotation of the left front wheel. The wheel speed sensor 10generally comprises a sensor rotor 104 adapted to rotate with thevehicle wheel, and a sensor assembly 102 fixedly secured to the shimportion 106 of the knuckle spindle 108. The sensor rotor 104 is fixedlysecured to a wheel hub 109 for rotation with the vehicle wheel.

As shown in FIG. 9, the sensor rotor 104 is formed with a plurality ofsensor teeth 120 at regular angular intervals. The width of the teeth120 and the grooves 122 therebetween are equal in the shown embodimentand define a unit angle of wheel rotation. The sensor assembly 102comprises a magnetic core 124 aligned with its north pole (N) near thesensor rate 104 and its south pole (S) distal from the sensor rotor. Ametal element 125 with a smaller diameter section 125a is attached tothe end of the magnetic core 124 nearer the sensor rotor. The free endof the metal element 125 faces the sensor teeth 120. An electromagneticcoil 126 encircles the smaller diameter section 125a of the metalelement. The electromagnetic coil 126 is adapted to detect variations inthe magnetic field generated by the magnetic core 124 to produce analternating-current sensor signal as shown in FIG. 10. That is, themetal element and the magnetic core 124 form a kind of proximity switchwhich adjusts the magnitude of the magnetic field depending upon thedistance between the free end of the metal element 125 and the sensorrotor surface. Thus, the intensity of the magnetic field fluctuates inrelation to the passage of the sensor teeth 120 and accordingly inrelation to the angular velocity of the wheel.

It should be appreciated that the wheel speed sensor 12 for the rightfront wheel has the substantially the same structure as the set forthabove. Therefore, explanation of the structure of the right wheel speedsensor 12 will be omitted in order to avoid unnecessary repetition inthe disclosure and in order to simplify the description.

FIG. 8 shows the structure of the rear wheel speed sensor 14. As withthe aforementioned left front wheel speed sensor 10, the rear wheelspeed sensor 14 comprises a sensor rotor 112 and a sensor assembly 102.The sensor rotor 112 is associated with a companion flange 114 which is,in turn, rigidly secured to a drive shaft 116 for rotation therewith.Thus, the sensor rotor 112 rotates with the drive shaft 116. The sensorassembly 102 is fixed to a final drive housing or a differential gearbox (not shown).

Each of the sensor assemblies applied to the left and right front wheelspeed sensors and the rear wheel sensor is adapted to output analternating-current sensor signal having a frequency proportional to orcorresponding to the rotational speed of the corresponding vehiclewheel. The electromagnetic coil 126 of each of the sensor assemblies 102is connected to the control module 200 to supply the sensor signalsthereto.

As set forth above, the control module 200 comprises the controller unit(FL) 202, the controller unit (FR) 204 and the controller unit (R) 206,each of which comprises a microcomputer. Therefore, the wheel speedsensors 10, 12 and 14 are connected to corresponding controller units202, 204 and 206 and send their sensor signals thereto. Since thestructure and operation of each of the controller units is substantiallythe same as that of the others, the structure and operation of only thecontroller unit 202 for performing the anti-skid brake control for thefront left wheel cylinder will be explained in detail.

FIG. 11 is a timing chart of the anti-skid control performed by thecontroller unit 202. As set forth above, the alternating-current sensorsignal output from the wheel speed sensor 10 is converted into arectangular pulse train, i.e. as the sensor pulse signal mentionedabove. The controller unit 202 monitors occurrences of sensor pulses andmeasures the intervals between individual pulses or between the firstpulses of groups of relatively-high-frequency pulses. Pulses are sogrouped that the measured intervals will exceed a predetermined value,which value will be hereafter referred to as "pulse interval threshold".

The wheel rotation speed V_(w) is calculated in response to each sensorpulse. As is well known, the wheel speed is generally inverselyproportional to the intervals between the sensor pulses, andaccordingly, the wheel speed V_(w) is derived from the interval betweenthe last sensor pulse input time and the current sensor pulse inputtime. A target wheel speed is designated V_(i) and the resultant wheelspeed is designated V_(w). In addition, the slip rate is derived fromthe rate of change of the wheel speed and an projected speed V_(v) whichis estimated from the wheel speed at the moment the brakes are appliedbased on the assumption of a continuous, linear deceleration withoutslippage. In general, the target wheel speed V_(i) is derived from thewheel speed of the last skid cycle during which the wheel decelerationrate was equal to or less than a given value which will be hereafterreferred to as "deceleration threshold a_(ref) ", and the wheel speed ofthe current skid cycle, and by estimating the rate of change of thewheel speed between wheel speeds at which the rate of deceleration isequal to or less than the deceleration threshold. In practice, the firsttarget wheel speed V_(i) is derived based on the projected speed V_(v)which corresponds to a wheel speed at the initial stage of brakingoperation and at which wheel deceleration exceeds a predetermined value,e.g. -1.2G, and a predetermined deceleration rate, for example 0.4G. Thesubsequent target wheel speed V_(i) is derived based on the projectedspeeds V_(v) in last two skid cycles. For instance, the decelerationrate of the target wheel speed V_(i) is derived from a difference of theprojected speeds V_(v) in the last two skid cycle and a period of timein which wheel speed varies from the first projected speed to the nextprojected speed. Based on the last projected speed and the decelerationrate, the target wheel speed in the current skid cycle is derived.

The acceleration and deceleration of the wheel is derived based on theinput time of three successive sensor pulses. Since the interval of theadjacent sensor signal pulses corresponds to the wheel speed, and thewheel speed is a function of the reciprocal of the interval, bycomparing adjacent pulse-to-pulse intervals, a value corresponding tothe variation or difference of the wheel speed may be obtained. Theresultant interval may be divided by the period of time of the intervalin order to obtain the wheel acceleration and deceleration at the unittime. Therefore, the acceleration or deceleration of the wheel isderived from the following equation: ##EQU1## where A, B and C are theinput times of the sensor pulses in the order given.

On the other hand, the slip rate R is a rate of difference of wheelspeed relative to the vehicle speed which is assumed as substantiallycorresponding to the target wheel speed. Therefore, in the shownembodiment, the target wheel speed V_(i) is taken as variable orparameter indicative of the assumed or projected vehicle speed. The sliprate R can be obtained by dividing a difference between the target wheelspeed V_(i) and the instantaneous wheel speed V_(w) by the target wheelspeed. Therefore, in addition, the slip rate R is derived by solving thefollowing equation: ##EQU2##

Finally, the controller unit 202 determines the control mode, i.e.,release mode, hold mode and application mode from the slip rate R andthe wheel acceleration or deceleration a_(w).

General operation of the controller unit 202 will be briefly explainedherebelow with reference to FIG. 11. Assuming the brake is applied at t₀and the wheel deceleration a_(w) exceeds the predetermined value, e.g.1.2 G at a time t₁, the controller unit 202 starts to operate at a timet₁. The first sensor pulse input time (t₁) is held in the controllerunit 202. Upon receipt of the subsequent sensor pulse at a time t₂, thewheel speed V_(w) is calculated by deriving the current sensor pulseperiod (dt=t₂ -t₁). In response to the subsequently received sensorpulses at times t₃, t₄ . . . , the wheel speed values V_(w2), V_(w3) . .. are calculated.

On the other hand, at the time t₁, the instantaneous wheel speed istaken as the projected speed V_(v). Based on the projected speed V_(v)and the predetermined fixed value, e.g. 0.4 G, the target wheel speedV_(i) decelerating at the predetermined deceleration rate 0.4 G isderived.

In anti-skid brake control, the braking force applied to the wheelcylinder is to be so adjusted that the peripheral speed of the wheel,i.e. the wheel speed, during braking is held to a given ratio, e.g. 85%to 80% of the vehicle speed. Therefore, the slip rate R has to bemaintained below a given ratio, i.e., 15% to 10%. In the preferredembodiment, the control system controls the braking force so as tomaintain the slip rate at about 15%. Therefore, a reference valueR_(ref) to be compared with the slip rate R is determined at a value at85% of the projected speed V_(v). As will be appreciated, the referencevalue is thus indicative of a slip rate threshold, which will behereafter referred to "slip rate threshold R_(ref) " throughout thespecification and claims, and varies according to variation of thetarget wheel speed.

In practical brake control operation performed by the preferredembodiment of the anti-skid control system according to the presentinvention, the electric current applied to the actuator attains alimited value, e.g., 2A to place the electromagnetic valve 30a in thehold mode as shown in FIG. 5 when the wheel speed remains inbetween thetarget wheel speed V_(i) and the slip rate threshold R_(ref). When theslip rate derived from the target wheel speed V_(i) and the wheel speedV_(w) becomes equal to or larger than the slip rate threshold R_(ref),then the supply current to the actuator 16 is increased to a maximumvalue, e.g. 5A to place the electromagnetic valve in the release mode asshown in FIG. 6. By maintaining the release mode, the wheel speed V_(w)is recovered to the target wheel speed. When the wheel speed is thusrecovered or resumed so that the slip rate R at that wheel speed becomesequal to or less than the slip rate threshold R_(ref), then the supplycurrent to the actuator 16 is dropped to the limited value, e.g. 2A toreturn the electromagnetic valve 30a to the hold mode. By holding thereduced fluid pressure in the wheel cylinder, the wheel speed V_(w) isfurther resumed to the target wheel speed V_(i). When the wheel speedV_(w) is resumed equal to or higher than the target wheel speed V_(i),the supply current is further dropped to zero for placing theelectromagnetic valve in the application mode as shown in FIG. 4. Theelectromagnetic valve 30a is maintained in the application mode untilthe wheel speed is decelerated at a wheel speed at which the wheeldeceleration becomes equal to or slightly more than the decelerationthreshold R_(ref) -1.2 G. At the same time, the projected speed V_(v) isagain derived with respect to the wheel speed at which the wheeldeceleration a_(w) becomes equal to or slightly larger than thedeceleration threshold a_(ref). From a difference of speed of the lastprojected speed and the instant projected speed and the period of timefrom a time obtaining the last projected speed to a time obtaining theinstant projected speed, a deceleration rate of the target wheel speedV_(i) is derived. Therefore, assuming the last projected speed isV_(v1), the instant projected speed is V_(v2), and the period of time isT_(v), the target wheel speed V_(i) can be obtained from the followingequation:

    V.sub.i =V.sub.v2 -(V.sub.v1 -V.sub.v2)/T.sub.v ×t.sub.e

where t_(e) is an elapsed time from the time at which the instantprojected speed V_(v2) is obtained.

Based on the input timing to t₁, t₂, t₃, t₄ . . . , deceleration ratea_(w) is derived from the foregoing equation (1). In addition, theprojected speed V_(v) is estimated as a function of the wheel speedV_(w) and rate of change thereof. Based on the instantaneous wheelspeeds V_(w1) at which the wheel deceleration is equal to or less thanthe deceleration threshold a_(ref) and the predetermined fixed value,e.g. 0.4 G for the first skid cycle of control operation, the targetwheel speed V_(i) is calculated. According to equation (2), the sliprate R is calculated, using successive wheel speed values V_(w1),V_(w2), V_(w3) . . . as parameters. The derived slip rate R is comparedwith the slip rate threshold R_(ref). As the wheel speed V_(w) dropsbelow the projected speed V_(v) at the time t₁, the controller unit 202switches the control mode from the application mode to the hold mode.Assuming also that the slip rate R exceeds the slip rate threshold atthe time t₄, then the controller unit 202 switches the control mode tothe release mode to release the fluid pressure at the wheel cylinder.

Upon release of the brake pressure in the wheel cylinder, the wheelspeed V_(w) recovers, i.e. the slip rate R drops until it is smallerthan the slip rate threshold at time t₇. The controller unit 202 detectswhen the slip rate R is smaller than the slip rate threshold R_(ref) andswitches the control mode from release mode to the hold mode.

By maintaining the brake system in the hold mode in which reduced brakepressure is applied to the wheel cylinder, the wheel speed increasesuntil it reaches the projected speed as indicated by the intersection ofthe dashed line (V_(v)) and the solid trace in the graph of V_(w) inFIG. 11. When the wheel speed V_(w) becomes equal to the target wheelspeed V_(i) (at a time t₈), the controller unit 202 switches the controlmode from the hold mode to the application mode.

As can be appreciated from the foregoing description, the control modewill tend to cycle through the control modes in the order applicationmode, hold mode, release mode and hold mode, as exemplified in theperiod of time from t₁ to t₈. This cycle of variation of the controlmodes will be referred to hereafter as "skid cycle". Practicallyspeaking, there will of course be some hunting and other minordeviations from the standard skid cycle.

The projected speed V_(v), which is meant to represent ideal vehiclespeed behavior, at time t₁ can be obtained directly from the wheel speedV_(w) at that time since zero slip is assumed. At the same time, thedeceleration rate of the vehicle will be assumed to be a predeterminedfixed value or the appropriate one of a family thereof, in order toenable calculation of the target wheel speed for the first skid cycleoperation. Specifically, in the shown example, the projected speed V_(v)at the time t₁ will be derived from the wheel speed V₁ at that time.Using the predetermined deceleration rate, the projected speed will becalculated at each time the wheel deceleration a_(w) in the applicationmode reaches the deceleration threshold a_(ref).

At time t₉, the wheel deceleration a_(w) becomes equal to or slightlylarger than the deceleration threshold a_(ref), then the secondprojected speed V_(v2) is obtained at a value equal to the instantaneouswheel speed V_(w) at the time t₉. According to the above-mentionedequation, the deceleration rate da can be obtained

    da=(V.sub.v1 -V.sub.v2)/(t.sub.9 -t.sub.1)

Based on the derived deceleration rate da, the target wheel speed V_(i)' for the second skid cycle of control operation is derived by:

    V.sub.i '=V.sub.v2 -da×t.sub.e

Based on the derived target wheel speed, the slip rate threshold B_(ref)for the second cycle of control operation is also derived. As will beappreciated from FIG. 11, the control mode will be varied during thesecond cycle of skid control operation, to hold mode at time t₉ at whichthe wheel deceleration reaches the deceleration threshold a_(ref) as setforth above, to release mode at time t₁₀ at which the slip rate Rreaches the slip rate threshold R_(ref), to hold mode at time t₁₁ atwhich the slip rate R is recovered to the slip rate threshold R_(ref),and to application mode at time t₁₂ at which the wheel speed V_(w)recovered or resumed to the target wheel speed V_(i) '. Further, itshould be appreciated that in the subsequent cycles of the skid controloperations, the control of the operational mode of the electromagneticvalve as set forth with respect to the second cycle of controloperation, will be repeated.

Relating the above control operations to the structure of FIGS. 3through 6, when application mode is used, no electrical current isapplied to the actuator of the electromagnetic valve 16a so that theinlet port 16b communicates with the outlet port 16c, allowing fluidflow between the pressure passage 42 and the brake pressure line 46. Alimited amount of electrical current (e.g. 2 A) is applied at times t₁,t₇, t₉ and t₁₁, so as to actuate the electromagnetic valve 16a to itslimited stroke position by means of the actuator 16, and the maximumcurrent is applied to the actuator as long as the wheel speed V_(w) isnot less than the projected speed and the slip rate is greater than theslip rate threshold R_(ref). Therefore, in the shown example, thecontrol mode is switched from the application mode to the hold mode attime t₁ and then to the release mode at time t₄. At time t₇, the sliprate increases back up to the slip rate threshold R_(ref), so that thecontrol mode returns to the hold mode, the actuator driving theelectromagnetic valve 16a to its central holding position with thelimited amount of electrical current as the control signal. When thewheel speed V_(w) finally returns to the level of the target wheel speedV_(i) at time t₈, the actuator 16 supply current is cut off so that theelectromagnetic valve 16a returns to its rest position in order toestablish fluid communication between the pressure line 42 and thebraking pressure line 46 via inlet and outlet ports 16b and 16c.

Referring to FIG. 12, the controller unit 202 includes an inputinterface 230, CPU 232, an output interface 234, RAM 236 and ROM 238.The input interface 230 includes an interrupt command generator 229which produces an interrupt command in response to every sensor pulse.In ROM, a plurality of programs including a main program (FIG. 13), aninterrupt program (FIG. 15), an sample control program (FIG. 19), atimer overflow program (FIG. 20) and an output calculation program (FIG.23) are stored in respectively corresponding address blocks 244, 246,250, 252 and 254.

The input interface also has a temporary register for temporarilyholding input timing for the sensor pulses. RAM 236 similarly has amemory block holding input timing for the sensor pulses. The contents ofthe memory block 240 of RAM may be shifted whenever calculations of thepulse interval, wheel speed, wheel acceleration or deceleration, targetwheel speed, slip rate and so forth are completed. One method ofshifting the contents is known from the corresponding disclosure of theU.S. Pat. No. 4,408,290. The disclosure of the U.S. Pat. No. 4,408,290is hereby incorporated by reference. RAM also has a memory block 242 forholding pulse intervals of the input sensor pulses. The memory block 242is also adapted to shift the contents thereof according to the mannersimilar to set forth in the U.S. Pat. No. 4,408,290.

An interrupt flag 256 is provided in the controller unit 202 forsignalling interrupt requests to the CPU. The interrupt flag 256 is setin response to the interrupt command from the interrupt commandgenerator 229. A timer overflow interrupt flag 258 is adapted to set anoverflow flag when the measured interval between any pair of monitoredsensor pulses exceeds the capacity of a clock counter.

In order to time the arrival of the sensor pulses, a clock is connectedto the controller unit 202 to feed time signals indicative of elapsedreal time. The timer signal value is latched whenever a sensor pulse isreceived and stored in either or both of the temporary register 231 inthe input interface 230 and the memory block 240 of RAM 236.

The operation of the controller unit 202 and the function of eachelements mentioned above will be described with reference to FIGS. 13 to30.

FIG. 13 illustrates the main program for the anti-skid control system.Practically speaking, this program will generally be executed as abackground job, i.e. it will have a lower priority than most otherprograms under the control of the same processor. Its first step 1002 isto wait until at least one sample period, covering a single sensor pulseor a group thereof, as described in more detail below, is completed asindicated when a sample flag FL has a non-zero value. In subsequent step1004, the sample flag FL is checked for a value greater than one, whichwould indicate the sample period is too short. If this is the case,control passes to a sample control program labelled "1006" in FIG. 13but shown in more detail in FIG. 19. If FL=1, then the control processis according to plan, and control passes to a main routine explainedlater with reference to FIG. 15. Finally, after completion of the mainroutine, a time overflow flag OFL is reset to signify successfulcompletion of another sample processing cycle, and the main programends.

FIG. 14 shows the interrupt program stored in the memory block 246 ofROM 238 and executed in response to the interrupt command generated bythe interrupt command generator 229 whenever a sensor pulse is received.It should be noted that a counter value NC of an auxiliary counter 233is initially set to 1, a register N representing the frequency dividerratio is set at 1, and a counter value M of an auxiliary counter 235 isset at -1. After starting execution of the interrupt program, thecounter value NC of the auxiliary counter 233 is decremented by 1 at ablock 3002. The auxiliary counter value NC is then checked at a block3004 for a value greater than zero. For the first sensor pulse, sincethe counter value NC is decremented by 1 (1-1=0) at the block 3002 andthus is zero, the answer of the block 3004 is NO. In this case, theclock counter value t is latched in a temporary register 231 in theinput interface 230 at a block 3006. The counter value NC of theauxiliary counter 233 is thereafter assigned the value N in a register235, which register value N is representative of frequency dividingratio determined during execution of the main routine explained later,at a block 3008. The value M of an auxiliary counter 235 is thenincremented by 1. The counter value M of the auxiliary counter 235labels each of a sequence of sample periods covering an increasingnumber of sensor pulses. After this, the sample flag FL is incrementedby 1 at a block 3012. After the block 3012, interrupt program ends,returning control to the main program or back to block 3002, whichevercomes first.

On the other hand, when the counter value NC is non-zero when checked atthe block 3004, this indicates that not all of the pulses for thissample period have been received, and so the interrupt program endsimmediately.

This interrupt routine thus serves to monitor the input timing t of eachpulse sampling period, i.e. the time t required to receive NC pulses,and signals completion of each sample period (M=0 through M=10, forexample) for the information of the main program.

Before describing the operation in the main routine, the general methodof grouping the sensor pulses into sample periods will be explained tofacilitate understanding of the description of the operation in the mainroutine.

In order to enable the controller unit 202 to accurately calculate thewheel acceleration and deceleration a_(w), it is necessary that thedifference between the pulse intervals of the single or grouped sensorpulses exceeding a given period of time, e.g. 4 ms. In order to obtainthe pulse interval difference exceeding the given period of time, 4 ms,which given period of time will be hereafter referred to as "pulseinterval threshold S", some sensor pulses are ignored so that therecorded input timing t of the sensor pulse groups can satisfy thefollowing formula:

    dT=(C-B)-(B-A)≧S (4 ms.)                            (3)

where A, B and C are the input times of three successive sensor pulsegroups.

The controller unit 202 has different sample modes, i.e. MODE 1, MODE 2,MODE 3 and MODE 4 determining the number of sensor pulses in each sampleperiod group. As shown in FIG. 16, in MODE 1 every sensor pulse inputtime is recorded and therefore the register value N is 1. In MODE 2,every other sensor pulse is ignored and the register value N is 2. InMODE 3, every fourth sensor pulse is monitored, i.e. its input time isrecorded, and the register value N is 4. In MODE 4, every eighth sensorpulse is sampled and the register value N is then 8.

The controller unit 202 thus samples the input timing of threesuccessive sensor pulses to calculate the pulse interval difference dTwhile operating in MODE 1. If the derived pulse interval difference isequal to or greater than the pulse interval threshold S, then sensorpulses will continue to be sampled in MODE 1. Otherwise, the inputtiming of every other sensor pulse is sampled in MODE 2 and from thesampled input timing of the next three sensor pulses sampled, the pulseinterval difference dT is calculated to again be compared with the pulseinterval threshold S. If the derived pulse interval difference is equalto or greater than the pulse interval threshold S, we remain in MODE 2.Otherwise, every four sensor pulses are sampled in MODE 3. The inputtimings of the next three sampled sensor pulses are processed to derivethe pulse interval difference dT. The derived pulse interval differencedT is again compared with the pulse interval threshold S. If the derivedpulse interval difference is equal to or greater than the pulse intervalthreshold S, the MODE remains at 3 and the value N is set to 4. On theother hand, if the derived pulse interval difference dT is less than thepulse interval threshold S, the mode is shifted to MODE 4 to sample theinput timing of every eighth sensor pulse. In this MODE 4, the value Nis set at 8.

For instance, in FIG. 16, the sensor pulses A₁, B₁ and C₁ are sampledunder MODE 1. In MODE 2, the sensor pulses a₁ and c₁ are ignored and thesensor pulses A₁ (=A₂), B₂ (=b₁) and C₂ (=b₂ =a₃) are sampled. In MODE3, the three sensor pulses c₂ (=b₃ =a₄), c₃ (=b₄) and c₄ following B₃(=c₂) are ignored and the sensor pulses A₃ (=A₁ =A₂), B₃ (=b₂ =a₃) andC₃ (=b₅ =a₆) are sampled. In MODE 4, the seven sensor pulses c₅ (=b₆=a₇), c₆ (=b₇ =a₈), c₇ (=b₈ =a₉), c₈ (=b₉ =a₁₀), c₉ (=b₁₀ =a₁₁), c₁₀(=b₁₁) and c₁₁ following B₄ (=c₃) are ignored and the sensor pulses A₄(=A₁ =A₂ =A₃), B₄ (=C₃ =b₅ =a₆) and C₄ are sampled.

Referring to FIG. 15, the main routine serves to periodically derive anupdated wheel acceleration rate value a_(w). In general, this is done bysampling larger and larger groups of pulses until the difference betweenthe durations of the groups is large enough to yield an accurate value.In the main routine, the sample flag FL is reset to zero at a block2001. Then the counter value M of the auxiliary counter 233, indicatingthe current sample period of the current a_(w) calculation cycle, isread out at a block 2002 to dictate the subsequent program steps.

Specifically, after the first sample period (M=.0.), the input timing ttemporarily stored in the temporary register 231 corresponding to thesensor pulse number (M=0) is read out and transferred to a memory block240 of RAM at a block 2004, which memory block 240 will be hereafterreferred to as "input timing memory". Then control passes to the block1008 of the main program. When M=2, the corresponding input timing t isread out from the temporary register 231 and transferred to the inputtiming memory 240 at a block 2006. Then, at a block 2008, a pulseinterval Ts between the sensor pulses of M=1 is derived from the twoinput timing values in the input timing memory 140. That is, the pulseinterval of the sensor pulse (M=1) is derived by:

    Ts=t.sub.1 -t.sub.0

where

t₁ is input time of the sensor pulse M1;

and

t₀ is input time of the sensor pulse M0.

The derived pulse interval T_(s) of the sensor pulse M1 is then comparedwith a reference value, e.g. 4ms., at a block 2010. If the pulseinterval T_(s) is shorter than the reference value, 4 ms., controlpasses to a block 2012 wherein the value N and the pulse interval T_(s)are multiplied by 2. The doubled timing value (2T_(s)) is again comparedwith the reference value by returning to the block 2010. The blocks 2010and 2012 constitute a loop which is repeated until the pulse interval(2nT_(s)) exceeds the reference value. When the pulse interval (2nT_(s))exceeds the reference value at the block 2010, a corresponding value ofN (2N) is automatically selected. This N value represents the number ofpulses to be treated as a single pulse with regard to timing.

After setting the value of N and thus deriving the sensor pulse groupsize then the auxiliary counter value NC is set to 1, at a block 2016.The register value N is then checked for a value of 1, at a block 2018.If N=1, then the auxiliary counter value M is set to 3 at a block 2020and otherwise control returns to the main program. When the registervalue N equals 1, the next sensor pulse, which would normally beignored, will instead be treated as the sensor pulse having the sampleperiod number M=3.

In the processing path for the sample period number M=3, thecorresponding input timing is read from the corresponding address of thetemporary register 231 and transferred to the input timing memory 240,at a block 2024. The pulse interval T₂ between the sensor pulses at M=1and M=3 is then calculated at a block 2026. The derived pulse intervalT₂ is written in a storage section of a memory block 242 of RAM 236 fora current pulse interval data, which storage section will be hereafterreferred at as "first pulse interval storage" and which memory block 242will be hereafter referred to as "pulse interval memory". After theblock 2026, control returns to the main program to await the next sensorpulse, i.e. the sensor pulse for sample period number M=4.

When the sensor pulse for M=4 is received, the value t of the temporaryregister 231 is read out and transferred to the input timing memory 240at block 2028. Based on the input timing of the sensor pulses for M=3and M=4, the pulse interval T₃ is calculated at a block 2030. The pulseinterval T₃ derived at the block 2030 is then written into the firstpulse interval storage of the pulse interval memory. At the same time,the pulse interval data T₂ previously stored in the first pulse intervalstorage is transferred to another storage section of the pulse intervalmemory adapted to store previous pulse interval data. This other storagesection will be hereafter referred to as "second pulse intervalstorage". Subsequently, at a block 2032 the contents of the first andsecond storages, i.e. the pulse interval data T₂ and T₃ are read out.Based on the read out pulse interval data T₂ and T₃, a pulse intervaldifference dT is calculated at block 2032 and compared with the pulseinterval threshold S to determine whether or not the pulse intervaldifference dT is large enough for accurate calculation of wheelacceleration or deceleration a_(w). If so, process goes to the block2040 to calculate the wheel acceleration or deceleration according tothe equation (1). The register value N is then set to 1 at the block2044 and thus MODE 1 is selected. In addition sample period number M isreset to -1, and the a_(w) derivation cycle starts again. On the otherhand, if at the block 2032 the pulse interval difference dT is too smallto calculate the wheel acceleration or deceleration a_(w), then thevalue N is multiplied by 2 at a block 2034. Due the updating of thevalue N, the sample mode of the sensor pulses is shifted to the nextmode.

When the block 2034 is performed and thus the sample mode is shifted toMODE 2 with respect to the sensor pulse of M=4', the sensor pulse c₂input following to the sensor pulse of M=4' is ignored. The sensor pulsec₃ following to the ignored sensor pulse c₂ is then taken as the sensorpulse to be sampled as M=3". At this time, the sensor pulse of M=4' istreated as the sensor pulse of M=2" and the sensor pulse of M=2 istreated as the sensor pulse of M=1". Therefore, calculation of theinterval difference dT and discrimination if the derived intervaldifference dT is greater than the pulse interval threshold S in theblock 2032 will be carried out with respect to the sensor pulse c₃ whichwill be treated as the sensor pulse of M=4". The blocks 2032 and 2034are repeated until the interval difference greater than the pulseinterval threshold S is obtained. The procedure taken in each cycle ofrepetition of the blocks 2032 and 2034 is substantially same as that setforth above.

As set forth above, by setting the counter value NC of the auxiliarycounter 233 to 1 at the block 2016, the input timing of the sensor pulsereceived immediately after initially deriving the sample mode at theblocks 2010 and 2012 will be sampled as the first input timing to beused for calculation of the wheel acceleration and deceleration. Thismay be contrasted with the procedure taken in the known art.

FIG. 18 shows timing of calculation of the wheel acceleration anddeceleration in comparison with the calculation timing of the wheelacceleration and deceleration in the prior art. As will be appreciatedfrom FIG. 18, in the prior art, after deriving the sample mode so thatthe pulse interval T_(s) is longer than the reference value, e.g. 4 ms,the first sensor pulse A' is sampled after thinning the correspondingnumber of sensor pulses e.g. 3 sensor pulses in the shown case. On theother hand, the first sensor pulse A, according to the presentinvention, can be sampled with respect to the sensor pulse inputimmediately after deriving the sample mode. As will be appreciatedherefrom, sample timing according to the present invention is fasterthan that in the prior art so that calculation of the wheel accelerationand deceleration can be performed at an earlier timing than that in theconventional art. In other words, the time lag of wheelacceleration/deceleration calculation due to sensor pulse grouping canbe shortened.

FIG. 17 shows a modified procedure can be taken for obtaining theinterval difference dT larger than the pulse interval threshold S. Inthis modification, SUB-MODE as illustrated is used instead of performingthe block 2034 to shift the sample mode of FIG. 16.

In this modification, when MODE 3 is selected during execution of themain routine of FIG. 15, at the blocks 2010, 2012 and 2016, thecontroller unit 202 is operating under MODE 3, first the sensor pulsesA₁, B₁ and C₁ are sampled as shown in FIG. 17. The pulse intervaldifference between (C₁ -B₁) and B₁ -A₁) is calculated in response to thesensor pulse C₁ (M=4). This operation for detecting the intervaldifference dT larger than the pulse interval threshold S issubstantially corresponding to the operation of the blocks 2032, in themain routine of FIG. 15. If the determined pulse interval difference dTis equal to or greater than the pulse interval threshold S, the wheelacceleration or deceleration will be calculated using the derived pulseinterval difference dT (SUM-MODE 1) at the block 2040 of the mainroutine of FIG. 15. On the other hand, if the derived pulse intervaldifference dT is less than the pulse interval threshold S, the sensorpulses A₂ (=A₁ :M=2), B₂ (-C₁ :M=4) and C₂ (M=6) are sampled in SUB-MODE2. If the pulse interval difference derived from the input timing of A₂,B₂ and C₂ is less than the pulse interval threshold S, then thecontroller unit 202 shifts the operation mode into SUB-MODE 3 in whichthe sensor pulses A₃ (=A₁ '=A₂ :M=2), B₃ (=C₂ :M=6) and C₃ (M=10) aresampled.

In both of SUB-MODEs 2 and 3, calculation for deriving the wheelacceleration or deceleration a_(w) relative to the sensor pulses M5, M6,M7, M8 and M9 are performed with taking the input timing of twoproceeding sensor pulses similarly to the procedure performed atSUB-MODE 1 when the interval difference dT larger than the pulseinterval threshold S can be detected with respect to M7, M8 or M9, forexample.

As will be appreciated herefrom, the SUB-MODE referred to hereabovemeans further variations of the sensor pulse sample mode in order toobtain the interval difference dT greater than the pulse intervalthreshold S for enabling calculation of the wheel acceleration anddeceleration at the block 2040 of the main routine of FIG. 15. With theforegoing modification of FIG. 17, even when the interval difference dTgreater than the pulse interval threshold S is obtained with respect tothe sensor pulse which has to be thinned i.e., omitted, under theprocedure of FIG. 16, the wheel acceleration and deceleration can bederived for reducing loss time. Further, according to this modifiedprocedure, the calculation timing of the wheel acceleration anddeceleration can follow a relatively abrupt change of the wheel spaced.

FIG. 19 shows the sample control program stored in the memory block 250of ROM 238. This sample control program is executed when the sample flagFL reaches a predetermined value. In the embodiment shown, the samplecontrol program is executed when the sample flag value FL equals 2. Whenthe sample flag value FL=2 at the block 1004 in FIG. 13, then the samplecontrol program is executed to multiply the auxiliary counter value N by2, at a block 4002 of FIG. 19. At the same time, the auxiliary countervalue NC is set to 1. Thereafter, the sample flag is reset to zero at ablock 4004.

The sample control program of FIG. 19 provides a quick and simpleadjustment of the sampling mode for both initial start-up and caseswhere the wheel accelerates so quickly that two sampling periods arecompleted within a single acceleration rate a_(w) derivation cycle.Setting N equal to 2N in block 4002 doubles the sample size and soeffectively doubles the sample period and setting NC to 1 ensures thatthe sampling will restart immediately with the next sensor pulse.

FIG. 20 shows the timer overflow program stored in the memory block 252of ROM. As set forth above, the clock counter 259 used in the embodimentshown has the capacity to count the clock pulses from the clockgenerator 11 for 64 ms. Therefore, the timer overflow program isexecuted as an interrupt program whenever the counter value of the clockcounter 259 reaches its maximum value (counter is full counted), i.e.every 64 ms. Upon starting execution of the timer overflow program, thetimer overflow value OFL is incremented by 1, at a block 4010. Theoverflow value OFL is then checked at a block 4012. If the overflowvalue OFL is less than a given value, e.g. 2, then control returns tothe main routine of the main program. Since the timer overflow value OFLis cleared at the end of the main program at the block 1008, if thetimer overflow program is executed twice during one cycle of executionof main program, the overflow value OFL would become 2. In this case,the answer at the block 4012 would be YES and the wheel speed valueV_(w) would be set to zero and the wheel acceleration and decelerationvalue a_(w) would also be set to zero.

For instance, if three successive sensor pulses are produced within theperiod of time for which the clock counter 259 counts the clock pulsesfrom the clock signal generator, as shown in FIG. 21, the input timingof respective sensor pulses may be as shown at A, B and C, correspondingto the counter values C_(A), C_(B) and C_(C). The overflow value OFLremains at zero in response to each of the sensor pulses A, B and C,since the sensor pulses are received before the counter time elapses.Therefore, the first time the timer overflow program is executed afterreceiving the sensor pulse C, the timer overflow value is incremented by1 during execution of the timer overflow program at the block 4010. Inthis case, the timer overflow value OFL is still only 1 which is smallerthan the limit checked for at the block 4012. On the other hand, if thesensor pulses are produced at intervals relatively long so that thetimer overflow program can be executed twice before three successivesensor pulses are sampled, as shown in FIG. 23, then the wheel is movelyso slowly that wheel acceleration a_(w) can not be reliably calculated.

Therefore, in the timer overflow program, as shown in FIG. 20, the wheelspeed V_(w) and the wheel acceleration or deceleration a_(w) are set tozero at the block 4014. By setting both the wheel speed V_(w) and thewheel acceleration and deceleration a_(w) to zero, serious errors willbe avoided.

FIG. 23 shows the output program for deriving the wheel speed V_(w),wheel acceleration and deceleration a_(w) and slip rate R, selecting theoperational mode, i.e. application mode, hold mode and release mode andoutputting an inlet signal EV and/or an outlet signal AV depending uponthe selected operation mode of the actuator 16.

When the application mode is selected the inlet signal EV goes HIGH andthe outlet signal EV goes HIGH. When the release mode is selected, theinlet signal EV goes LOW and the outlet signal AV also goes LOW. Whenthe selected mode is the hold mode, the inlet signal EV remains HIGHwhile the outlet signal AV goes LOW. These combinations of the inletsignal EV and the outlet signal AV correspond to the actuator supplycurrent levels shown in FIG. 11 and thus actuate the electromagneticvalve to the corresponding positions illustrated in FIGS. 4, 5 and 6.

The output program is stored in the memory block 254 and adapted to beread out periodically, e.g. every 10 ms, to be executed as an interruptprogram. The output calculation program is executed in the time regionsshown in hatching in FIGS. 24 and 25.

During execution of the output calculation program, the pulse interval Tis read out from a memory block 241 of RAM which stores the pulseinterval, at a block 5002. As set forth above, since the pulse intervalT is inversely proportional to the wheel rotation speed V_(w), the wheelspread can be derived by calculating the reciprocal (1/T) of the pulseinterval T. This calculation of the wheel speed V_(w) is performed at ablock 5004 in the output program. After the block 5004, the target wheelspeed V_(i) is calculated at a block 5006. The manner of deriving thetarget wheel speed V_(i) has been illustrated in the U.S. Pat. Nos.4,392,202 to Toshiro MATSUDA, issued on July 5, 1983, 4,384,330 also toToshiro MATSUDA, issued May 17, 1983 and 4,430,714 also to ToshiroMATSUDA, issued on Feb. 7, 1984, which are all assigned to the assigneeof the present invention. The disclosure of the above-identified threeU.S. patents are hereby incorporated by reference for the sake ofdisclosure. In addition, the method of deriving the wheel speed V_(w)may be appreciated from FIG. 30. As is bvious herefrom, the target wheelspeed V_(i) is derived as a function of wheel speed deceleration asactually detected. For instance, the wheel speed V_(w) at (a) of FIG. 30at which the wheel deceleration a_(w) exceeds the deceleration thresholda_(ref), e.g. -1.2 G is taken as one reference point for deriving thetarget wheel speed V_(i). The wheel speed at (b) of the current skidcycle, at which the wheel deceleration a_(w) also exceeds thedeceleration threshold a_(ref), is taken as the other reference point.In addition, the period of time between the points a and b is measured.Based on the wheel speed V_(w1) and V_(w2) and the measured period P,the deceleration rate dV_(i) is derived from:

    dV.sub.i =(V.sub.w1 -V.sub.w2)/P                           (4)

This target wheel speed V_(i) is used for skid control in the next skidcycle.

It should be appreciated that in the first skid cycle, the target wheelspeed V_(i) cannot be obtained. Therefore, for the first skid cycle, apredetermined fixed value will be used as the target wheel speed V_(i).

At a block 5008 (FIG. 23), the slip rate R is calculated according tothe foregoing formula (2). Subsequently, the operational mode isdetermined on the basis of the wheel acceleration and deceleration a_(w)and the slip rate R, at a block 5010. FIG. 26 shows a table used indetermining or selecting the operational mode of the actuator 16 andwhich is accessed according to the wheel acceleration and decelerationa_(w) and the slip rate R. As can be seen, when the wheel slip rate R isin the range of 0 to 15%, the hold mode is selected when the wheelacceleration and deceleration a_(w) is lower than -1.0 G and theapplication mode is selected when the wheel acceleration anddeceleration a_(w) is in the range of -1.0 G to 0.6 G. On the otherhand, when the slip rate R remains above 15%, the release mode isselected when the wheel acceleration and deceleration a_(w) is equal toor less than 0.6 G, and the hold mode is selected when the wheelacceleration and deceleration is in a range of 0.6 G to 1.5 G. When thewheel acceleration and deceleration a_(w) is equal to or greater than1.5 G, the application mode is selected regardless of the slip rate.

According to the operational mode selected at the block 5010, the signallevels of the inlet signal EV and the outlet signal AV are determined sothat the combination of the signal levels corresponds to the selectedoperation mode of the actuator 16. The determined combination of theinlet signal EV and the outlet signal AV are output to the actuator 16to control the electromagnetic valve.

It should be appreciated that, although the execution timing of theoutput calculation program has been specified to be about 10 ms in theforegoing disclosure, the timing is not necessarily fixed to thementioned timing and may be selectable from the approximate range of 1ms to 20 ms. The execution timing of the output program is fundamentalyto be determined in accordance with the response characteristics of theactuator.

Although the pulse intervals have been described as being the valuescalculated at the blocks 2026, 2030 . . . of the main routine of FIG. 15in the foregoing embodiment, it would be possible to performcalculations for deriving the wheel speed V_(w) at the correspondingblocks as shown by the blocks 2026' and 2030' of FIG. 27. In this case,the derived wheel speed V_(w) may be stored in an appropriate address ofRAM. According to flowchart of FIG. 27, the wheel speed V_(w1) iscalculated with respect to the sensor pulse M=2 and wheel speed V_(w2)is calculated with respect to the sensor pulse M=3. Specifically, thewheel speed is calculated whenever the sensor pulse to be sampled isreceived, as shown in FIG. 28. In this modification, even if the pulseinterval difference is not large enough to allow calculation of thewheel acceleration and deceleration at the sensor pulse M=10, the delayin deriving the control output, i.e. the combination of the inlet signalEV and the outlet signal AV will be minimized by calculating the wheelspeed during execution of the main routine.

A modification is provided to the foregoing modification of FIG. 27 byneglecting the block 2034. In this case, sampling mode for sampling thesensor pulse input timing is not changed after it is once decided at theblock 2010 unless adjusted upon occasion at block 4002. Therefore,calculation of the wheel speed will be performed at the timingillustrated in FIG. 29.

FIG. 31 shows another embodiment of the controller unit 202 in thepreferred embodiment of the anti-skid control system according to thepresent invention. In practice, the circuit shown in FIG. 31 performsthe same procedure in controlling the actuator 16, and each block of thecircuit performs operation substantially corresponding to that performedby the foregoing computer flowchart.

In FIG. 31, the wheel speed sensor 10 is connected to a shaping circuit260 provided in the controller unit 202. The shaping circuit 260produces the rectangular sensor pulses having a pulse interval inverselyproportional to the wheel speed V_(w). The sensor pulse output from theshaping circuit 260 is fed to a pulse pre-scaler 262 which counts thesensor pulses to produce a sample command for sampling input timing whenthe counter value reaches a predetermined value. The predetermined valueto be compared with the counter value in the pulse pre-scaler 262 isdetermined such that the intervals between the pairs of three successivesample commands will be sufficiently different to allow calculation ofthe wheel acceleration and deceleration rate.

The sample command is fed to a flag generator 264. The flag generator264 is responsive to the sample command to produce a flag signal. Theflag signal of the flag generator 264 is fed to a flag counter 266 whichis adapted to count the flag signals and output a counter signal havinga value representative of its counter value.

At the same time, the sample command of the pulse pre-scaler 262 is fedto a latching circuit 268 which is adapted to latch the signal value ofa clock counter signal from a clock counter 267 counting the clock pulseoutput by a clock generator 11. The latched value of the clock countersignal is representative of the input timing of the sensor pulse whichactivates the pulse pre-scaler 262 to produce the sample command. Thelatching circuit 268 sends the input timing indicative signal having avalue corresponding to the latched clock counter signal value, to amemory controller 274. The memory controller 274 is responsive to amemory command input from an interrupt processing circuit 272 which inturn is responsive to the flag counter signal to issue a memory commandwhich activates the memory controller 274 to transfer the input timingindicative signal from the latching circuit 268 to a memory area 276.The memory 276 sends the stored input timing indicative signal to asample controller 270 whenever the input timing signal valuecorresponding to the latched value of the latching circuit 268 iswritten therein. The sample controller 270 performs operationssubstantially corresponding to that performed in the blocks 2008, 2010,2012, 2032 and 2034 in FIG. 15, i.e. it determines the number of sensorpulses in each group to be ignored. The sample controller 270 outputs apulse number indicative signal to the pulse pre-scaler 262, which pulsenumber indicative signal has a value approximating the predeterminedvalue to be compared with the counter value in the pulse pre-scaler 262.

The memory 276 also feeds the stored input timing indicative signal to awheel acceleration and deceleration calculation circuit 278 and a pulseinterval calculation circuit 280. The wheel acceleration anddeceleration calculation circuit 278 first calculates a pulse intervaldifference between pairs of three successively sampled sensor pulses.The obtained pulse interval difference is compared with a referencevalue so as to distinguish if the pulse interval difference is greatenough to allow calculation of the wheel acceleration and decelerationa_(w). If the obtained pulse interval difference is greater than thereference value, then the wheel acceleration and decelerationcalculation circuit 278 performs calculation of the wheel accelerationand deceleration according to the foregoing formula (1). If the obtainedpulse interval difference is smaller than the reference value, the wheelacceleration and deceleration calculation circuit 278 shifts theoperational mode thereof in a manner substantially corresponding to theprocedure disclosed with reference to FIG. 16, so as to achieve a pulseinterval difference large enough to permit the wheel acceleration anddeceleration calculation. On the other hand, the pulse intervalcalculation circuit 280 performs calculations to obtain the pulseinterval between the current pulse and the immediate preceding pulse andsends a pulse interval indicative signal to a memory 282. The memory 282sends a stored pulse interval indicative signal to a wheel speedcalculation circuit 284 which is associated with a 10 ms timer 292. The10 ms timer 292 produces a timer signal every 10 ms to activate thewheel speed calculation circuit 284. The wheel speed calculation circuit284 is responsive to the timer signal to perform calculation of thewheel speed V_(w) by calculating the reciprocal value of the pulseinterval indicative signal from the memory 282. The wheel speedcalculation circuit 284 thus produces a wheel speed indicative signal tobe fed to a target wheel speed calculation circuit 288 and to a sliprate calculation circuit 290 which is also associated with the 10 mstimer to be activated by the timer signal every 10 ms.

The target wheel speed calculation circuit 288 is adapted to detect thewheel speed V_(w) at which the wheel acceleration and deceleration a_(w)calculated by the wheel acceleration and deceleration calculatingcircuit 278 exceeds than a predetermined deceleration rate -b. Thetarget wheel speed calculation circuit 288 measures the interval betweentimes at which the wheel deceleration exceeds the predetermineddeceleration value. Based on the wheel speed at the foregoing times andthe measured period of time, the target wheel speed calculation circuit288 derives a decelerating ratio of the wheel speed to produce a targetwheel speed indicative signal. The target wheel indicative signal of thetarget wheel speed calculation circuit 288 and the wheel speedindicative signal from the wheel speed calculation circuit 284 are fedto a slip rate calculation circuit 290.

The slip rate calculation circuit 290 is also responsive to the timersignal from the 10 ms timer 292 to perform calculation of the slip rateR based on the wheel speed indicative signal from the wheel speedcalculation circuit 284 and the target wheel speed calculation circuit288, in accordance with the formula (2).

The slip rate calculation circuit 290 and the wheel acceleration anddeceleration calculation circuit 278 are connected to an output unit 294to feed the acceleration and deceleration indicative signal and the sliprate control signal thereto. The output unit 294 determines theoperation mode of the actuator 16 based on the wheel acceleration anddeceleration indicative signal value and the slip rate indicative signalvalue according to the table of FIG. 26. The output unit 294 thusproduces the inlet and outlet signals EV and AV with a combination ofsignal levels corresponding to the selected operation mode of theactuator.

On the other hand, the wheel speed calculation circuit 284 is alsoconnected to the flag counter 266 to feed a decrementing signal wheneverthe calculation of the wheel speed is completed and so decrement thecounter value of the flag counter by 1. The flag counter 266 is alsoconnected to a comparator 295 which is adapted to compare the countervalue of the flag counter with a reference value, e.g. 2. When thecounter value of the flag counter 266 is greater than or equal to thereference value, the comparator 295 outputs a comparator signal to anoverflow detector 296. The overflow detector 296 is responsive to thecomparator signal to feed a sample mode shifting command to be fed tothe pulse pre-scaler 262 to shift the sample mode to increase the numberof the sensor pulses in each sample group.

On the other hand, the clock counter 267 is connected to an overflowflag generator 297 which detects when the counter value reaches the fullcount of the clock counter to produce an overflow flag signal. Theoverflow signal of the overflow flag generator 297 is fed to an overflowflag counter 298 which is adapted to count the overflow flag signals andsend an overflow counter value indicative signal to a judgment circuit299. The judgment circuit 299 compares the overflow counter indicativesignal value with a reference value e.g. 2. The judgment circuit 299produces a reset signal when the overflow counter indicative signalvalue is equal to or greater than the reference value. The reset signalresets the wheel acceleration and deceleration calculation circuit 278and the wheel speed calculation circuit 284 to zero. On the other hand,the overflow flag counter is connected to the wheel speed calculationcircuit 284 and is responsive to the decrementing signal output from thewheel speed calculation circuit as set forth above to be reset inresponse to the decrementing signal.

As set forth above, according to the present invention, an anti-skidbrake control system can sample the input timing of sensor pulseswithout error. In addition, as will be appreciated, the inventionconstructed as hereabove, fulfills all of the objects and advantagessought therefor.

What is claimed is:
 1. An anti-skid brake control system for anautomotive brake system comprising:a hydraulic brake circuit including awheel cylinder for applying braking force to a vehicle wheel; a pressurecontrol valve disposed within said hydraulic circuit and operative toincrease fluid pressure in said wheel cylinder in a first positionthereof, to decrease the fluid pressure in said wheel cylinder in asecond position thereof and to hold the fluid pressure in said wheelcylinder constant in a third position thereof; a wheel speed sensormeans producing sensor signal pulses separated by intervalsrepresentative of the detected wheel speed; a timer means for producingtimer signals; a first means, responsive to said sensor signal pulses,for sampling values of said timer signals and for storing the sampledtimer signal values; a second means for reading out said stored timersignal values and processing the read-out timer signal values to producea control signal which actuates said pressure control valve to one ofsaid first, second and third positions so as to adjust the wheelrotational speed toward an optimal relationship with vehicle speed, saidsecond means performing at least one of a plurality of differentoperations based on said sampled timer signal values; a third means forproviding identification corresponding to each sampled timer signalvalue, said identification identifying which of said plurality ofdifferent operations is to be performed on the corresponding sampledtimer signal value; a fourth means responsive to said sensor signalpulses to interrupt operation of said second means and activate saidfirst and third means; and a fifth means responsive to theidentification provided by said third means for controlling said secondmeans to perform the operation identified by the identification withrespect to the corresponding sampled timer signal value.
 2. Theanti-skid brake control system as set forth in claim 1, wherein saidfirst means comprises means for sampling the timer signal value andmeans for storing the sampled timer signal value and, said second meansas controlled by said fifth means, performs one of the following basedon said identification corresponding to the sampled timer signal valuebeing processed;a first processing operation to transfer the sampledtimer signal value from said sampling means to said storing means; asecond processing operation to transfer the sampled timer signal valuefrom said sampling means to said storing means and calculate thedifference between the two most recently sampled timer signal values;and a third processing operation to transfer the sampled timer signalvalue from said sampling means to said storing means, to calculate thedifference between the two most recently sampled timer signal values,and to calculate a wheel acceleration or deceleration value on the basisof the difference between the two most recently calculated timer signalvalue differences.
 3. The anti-skid brake control system as set forth inclaim 2, wherein said third means provides a sequential number for eachsequentially sampled timer signal value as said identification.
 4. Theanti-skid control system as set forth in claim 3, wherein said thirdmeans provides said sequential number when said first, second and thirdprocessing operations are all completed.
 5. The anti-skid control systemas set forth in claim 3, wherein said first means is adapted to samplevalues of said timer signals at a given number of sensor signal pulses.6. The anti-skid brake control system as set forth in claim 5, whereinsaid first means determines said given number so that an interval oftime indicative of a difference between successively sampled timersignal values is greater than a predetermined value.
 7. The anti-skidcontrol system as set forth in claim 6, wherein said first means variessaid given number in step-by-step fashion until said difference becomesgreater than said predetermined value.
 8. The anti-skid control systemas set forth in claim 7, wherein said first means cyclically operates todetect said difference.
 9. The anti-skid control system as set forth inclaim 8, wherein said first means increase said given number by a givenvalue at each cycle of operation when said difference is smaller thansaid predetermined value.
 10. The anti-skid brake control system as setforth in claim 9, which further comprises a sixth means, responsive tothe interruption operation of said fourth means for counting occurrencesof interruption of said second means in an interruption counter eachtime an interruption occurs, and counting down the interruption by 1whenever said second means completes a cycle of operation.
 11. Ananti-skid brake control system for an automotive vehicle comprising:ahydraulic brake circit including wheel cylinders respectivelycorresponding to respective vehicle wheels for applying braking force tothe corresponding wheel; a pressure control valve associated with eachof said wheel cylinders for controlling fluid pressure to be applied tothe corresponding wheel cylinder, said pressure control valve beingoperative to increase the fluid pressure in said wheel cylinder at afirst position thereof, to decrease the fluid pressure in said wheelcylinder at a second position thereof and to maintain the fluid pressurein said wheel cylinder at a constant value at a third position thereof;a wheel speed sensor means for outputting sensor signal pulses havingpulse intervals respectively corresponding to instantaneous wheel speed;a timer means for outputting timer signals; a controller operative in asampling operation for receiving said sensor signal pulses and saidtimer signals for sampling values of said timer signals at a givennumber of sensor signal pulses, and, in an arithmetic operation, fordetermining instantaneous slip rate at the corresponding wheel andacceleration and deceleration of said corresponding wheel based on thesampled timer signal values for controlling the valve position of saidpressure control valve in a predetermined pattern for optimizing brakingcharacteristics, said arithmetic operation including a plurality ofdifferent processes, said controller interrupting said arithmeticoperation in response to said given member of sensor signal pulses toperform said sampling operation and said controller providing a label toeach sampled timer signal value for identifying one of said processes insaid arithmetic operation to be performed with respect to thecorresponding timer signal value.
 12. The anti-skid brake control systemas set forth in claim 11, wherein said controller determines said givennumber so that a difference of durations of time between successivelysampled timer signal values is greater than a predetermined value. 13.The anti-skid brake control system as set forth in claim 12, whereinsaid controller counts the occurrences of interruption of saidarithmetic operation and decrease the counted value each time saidarithmetic operation is completed.
 14. The anti-skid control system asset forth in claim 13, wherein said controller carries out said samplingoperation to omit one or more sensor signal pulses without sampling thecorresponding timer signal value so that said difference becomes greaterthan said predetermined value.
 15. The anti-skid control system as setforth in claim 12, wherein said controller varies said given number instep-by-step fashion until said difference becomes greater than saidpredetermined value.
 16. The anti-skid brake control system as set forthin claim 11, wherein said controller provides a sequential number foreach sequentially sampled timer signal value as said label.
 17. Theanti-skid control system as set forth in claim 16, wherein saidcontroller provides said sequential number when said arithmeticoperation is completed.
 18. The anti-skid control system as set forth inclaim 17, wherein said controller is operative in performing saidarithmetic operation to place said pressure control valve at said firstposition whenever the determined slip rate and wheel accelerationsatisfy a predetermined first condition for applying a brake, at saidsecond position whenever the determined slip rate and wheel decelerationsatisfies a predetermined second condition for releasing a brake, and atsaid third position whenever the determined slip rate and wheelacceleration or deceleration satisfies a predetermined third conditionfor holding the braking pressure at the constant value.
 19. Theanti-skid control system as set forth in claim 18, wherein saidcontroller is operative in performing said arithmetic operation toderive a wheel speed based on input time data of adjacent pairs ofsensor signal pulses and the wheel acceleration and deceleration basedon three sequentially sampled timer signal values.
 20. A method foranti-skid controlling an automotive hydraulic brake system including awheel cylinder and a pressure control valve, fluid pressure in saidwheel cylinder being increased when said pressure control valve isplaced at a first position thereof, is decreased as said pressurecontrol valve is placed at a second position thereof and is maintainedat substantially a constant value at a third position of said pressurecontrol valve, said method comprising the steps of:detecting wheelrotation speed to sequentially produce sensor signal pulses havingintervals between mutually adjacent sensor signal pulses representativeof wheel speed; sequentially producing timer signals; sampling values ofsaid timer signals in response to a given number of said sensor signalpulses to store said timer signal values as input time data of thecorresponding sensor signal pulses, and giving a label number to eachsampled input timer data as an identifying label thereof; adjusting saidgiven number to determine a sample mode such that an interval of timeindicative of a difference between successively sampled timer signalvalues is greater than a predetermined value; performing an arithmeticoperation for processing said input time data for detecting slip dataand wheel acceleration and deceleration for controlling said pressurecontrol valve positions according to a predetermined pattern foroptimizing braking characteristics at the vehicle wheel, said arithmeticoperation containing a plurality of different processes to be identifiedand performed in accordance with said label number; and interruptingsaid arithmetic operation in response to said given number of sensorsignal pulses for performing an operation of sampling the timer signalvalue and storing the sampled timer signal value as said input time datarelative to the input sensor signal.
 21. The method as set forth inclaim 20, which further comprises a step of giving said label number inresponse to completion of said arithmetic operation.
 22. The method asset forth in claim 21, wherein said plurality of difference processescomprise:a first process to transfer the input time data from a samplingdevice to a storage device; a second process to transfer the input timedata from said storage device to a calculating device and calculatingsaid difference; and a third process to calculate said difference andcalculate wheel acceleration and deceleration based on a furtherdifference of time between two most recently calculated differences. 23.The method as set forth in claim 22, which further comprises the step ofcounting occurrences of interruption of said arithmetic operation in acounter and decrementing the counter by 1 each time said arithmeticoperation is completed, and repeating said arithmetic operation untilsaid counter value becomes zero.
 24. The method as set forth in claim23, wherein said step of adjusting said given number is repeated toincrease the number of sensor signal pulses to be ignored withoutsampling the corresponding input time data until said difference becomesgreater than said predetermined value.